Essential oils should really be chosen by the individual oils specific chemistry. However, for beginners, it can be helpful to understand and choose an essential oil based on the chemical family.

Below are general descriptions of the ten most common chemical families in essential oils, common characteristics for each family and shelf life.

If you’re unclear about the meaning of a therapeutic property, you can go to my earlier blog post Glossary of Essential Oil Properties to look it up.

Ten Essential Oil Chemical Families and Their General Properties

MONOTERPENES
Monoterpene Properties: This is a huge chemical family, found in many essential oils. The monoterpenes are great airborne deodorizers, assist with skin penetration of a blend and can be rubefacient. They are my favorites to use in DIY cleaning products because they aren’t expensive and smell clean.
Monoterpene Safety: Essential oils with a high percentage of monoterpene components don’t have a long shelf life and are easily damaged by the presence of oxygen (making them oxidized). Once this happens, these oils can create skin problems such as irritation or skin sensitization. Use old monoterpene essential oils for cleaning. 
Monoterpene Shelf Life: Only 1-3 years. Keep in a cool dark location.

SESQUIPTERPENES:
Sesquiterpenes: This family isn’t easy to generalize so always check the GC/MS Report of the individual essential oil.  For the most part, the Sesquiterpene-rich essential oils tend to be a friend to the skin being both anti-inflammatory and cicatrizant. These oils are useful when addressing pain and are emotionally grounding.
Sesquiterpene Safety: Luckily for us, these beautiful essential oils don’t generally have safety concerns. One exception would be Black Pepper which may cause irritation if you have sensitive skin. According to Robert Tisserand in his book, “Essential Oil Safety,” there is a component found in Myrrh called b-elemene and it has shown antiangiogenic action, therefore, please do not use in pregnancy.
Sesquiterpenes Shelf Life: They have a nice long shelf life of 6-8 years or longer.

MONOTERPENOLS
Monoterpenol Properties: This is a lovely chemical family whose essential oils are great for skin nourishment and emotional balance. They can help heal infections and boost the immune system.
Monoterpenol Safety: Most of the essential oil high in Monoterpenol components are not known to be irritating. Of course, there’s always an exception, and in this case, it is Peppermint Essential Oil which may irritate the skin. Avoid Peppermint in children under the age of 5 and keep away from infants.
Monoterpenol Shelf Life: 3-5 years

SESQUITERPENOLS
Sesquiterpenols:  Like the Sesquiterpenes above, the Sesquiterpenol family is hard to generalize so be sure to check the GC/MS report of each individual essential oil. I love this family! The essential oils tend to be grounding, sedative, and anti-inflammatory. They have a shelf-life of 6-8 years or longer.
Sesquiterpenol Safety: Happily, the lovely oils in this chemical family are safe oils and not known to be skin irritants. Be mindful that the component b-eudesmol (found in Blue Cypress) is cautioned against anticoagulant drugs, peptic ulcer, or bleeding disorders and should be avoided during pregnancy due to the antiangiogenic action.
Sesquiterpenol Shelf Life: 6-8 years or longer.

ESTERS
Ester Properties:  Esters generally have a lovely fresh or floral aroma and favored by perfumers. Nearly all essential oils in this chemical family are wonderfully antispasmodic as well as calming and soothing for the CNS (central nervous system).
Ester Safety:  As long as you’re following appropriate dilutions on the skin, there are no real safety issues with essential oils high in esters. The one big exception is the component methyl salicylate which is present in Wintergreen and Birch.
Ester Shelf Life: The Ester-rich essential oils have a shelf-life of 3-5 years.

ALDEHYDES
Aldehyde Properties: This chemical family has essential oils that have a lovely lemony aroma. They are my go-to essential oils for fungus conditions and antispasmodic. Aldehydes are cooling for hot conditions.
Aldehyde Safety: Essential Oils with a high percentage of aldehyde molecules can be irritating to the skin, especially if they are oxidized (this can happen with older oils – see this post).  I recommend a low dilution when using aldehydes in the skin, specifically not more than 6 drops of aldehyde-rich essential oil in 1 ounce of carrier. Aldehydes are very susceptible to oxidation so remember to keep then cool, dark and full (no headspace at the top of the bottle).  According to Robert Tisserand, “People with estrogen-related cancers should avoid the internal use and high-dose, long-term skin use of oils high in citral. In such cases, oils high in citral can be used in 1% dilutions for short periods without concern.”
Aldehyde Shelf Life: 2 years. Do not keep these oils beyond this time frame. Use them for cleaning or just toss them. They can be highly problematic for skin.

PHENOLS
Phenol Properties: NEVER use undiluted on the skin. This family is anti-infectious and strong immune stimulants.
Phenol Safety: This is a great chemical family, but has important safety concerns. Many essential oils rich in phenol components are highly irritating to skin and mucous membranes. You only have to experience a bit of Clove or Oregano near the eye or mouth to understand this. Please use sparingly and for a short period of time (maximum 2 weeks to avoid skin sensitization).
Phenol Shelf Life: 3-5 years

KETONES
Ketone Properties:  Ketone-rich essential oils are great for pain and circulation.
Ketone Safety: Another family with some safety issues, in particular, camphor.
Ketone Shelf Life: 

OXIDES
Oxides (specifically 1,8 cineole): Strong respiratory oils, help relieve pain and mentally stimulating.
Oxide Safety:
Oxide Shelf Life:

ETHERS
Ether Properties:  Essential oil high in ether constituents are powerful antispasmodics. That said, I still try to use essential oils from the safe Ester family first.  Ethers can be very helpful for stomach issues.
Ether Safety: I include this chemical family last becuase it has significant safety issues. 
Ether Shelf Life: 3-5 years

 

Related Posts:

• AWP 035: Chemical Families Deep Dive – Part 2
• Essential Oil Chemical Families: A Beginners Guide
• AWP 034: Chemical Families Deep Dive – Part 1
• AWP 008: 5 Reasons Every Aromatherapist Should Learn Essential Oil Chemistry
• 5 Tips for Enhancing your Workplace with Essential Oils